Wide band-gap GaN and related materials are recognized to be among the most attractive compound semiconductors for use in a variety of devices. They are adapted for optoelectronic and microelectronic devices which operate in a wide spectral range, from visible to ultraviolet and in the high temperature/high power applications area. The main advantages of nitride semiconductors in comparison with other wide-band-gap semiconductors is their low propensity to degrade at high temperature and high power when used for optical and microelectronic devices. Meanwhile, low-dimensional quantum confinement effects (i.e. in quantum wires and dots) are expected to become one of the foremost technologies for improving optical device performances. Fabrication of a variety of low-dimensional structures in III-V nitrides has been undertaken using methods such as etching, re-growth, overgrowth on selected areas, growth on tilted substrates, self-organization process, etc.
Despite the technological advances of the last few years, one of the key obstacles preventing further developments in GaN devices is the lack of high quality and commercially available low-cost, free-standing GaN substrates. Alternative substrates, such as sapphire and SiC, are commonly employed in nitride-based devices. As a result of lattice mismatch and large differences in the thermal expansion coefficients between the deposited film and substrate (heteroepitaxy), a very high (109 to 1010 cm−2) density of threading dislocations and serious wafer bending/cracking, induced by undesired residual strain, occurs in the grown nitride layers. These factors can significantly affect the performance and lifetime of nitride-based optoelectronic and microelectronic devices.
Epitaxial lateral overgrowth technique (so-called ELOG and its modifications: facet initiated epitaxial lateral overgrowth (FIELO) and Pendeo (from the Latin to hang or be suspended)) is the most widely used approach employed for suppressing bending and a significant fraction of the threading dislocations in the material. Laterally overgrowing oxide (or metal) stripes deposited on initially-grown GaN films has been shown to achieve about two orders of magnitude reduction in the dislocation density, reducing it to the 107 cm−2 level. However, the low defect-density material only occurs in the wing region, located in the coalescence front, and represents only approximately one fifth of the whole wafer surface area. Large coalescence front tilting and tensile stress are both present in the overgrowth region.
Low defect-density free standing GaN is currently one of the materials of choice to achieve the desired specification for optoelectronic and microelectronic devices. Bulk (melt or sublimation) and hydride vapour phase epitaxy (HVPE) are the two main techniques for growing free standing and low defect-density GaN. Bulk GaN growth techniques operating at a very high pressure of ˜15 kbar have been successful in growing low dislocation density (<107 cm−2) material. Unfortunately, this technology suffers from a low growth rate and is limited to small diameter substrates, making them very expensive and uneconomic for commercial manufacturing. The record nitride laser lifetime of 15,000 hours under CW-operation at the 30 mW output level has recently been demonstrated by Nichia Chemicals Inc., using the HVPE grown substrate. HVPE is clearly one of the most promising techniques available to provide low defect-density GaN and large diameter commercial free-standing GaN substrates.
HVPE is a reversible equilibrium-based hot wall process with several advantages: (1) high growth rate (up to 100 μm/hr—more than 100 times faster than that of the MOCVD and MBE methods; (2) low running costs; (3) the mutual annihilation of mixed dislocations lowers the defect densities in thick GaN. However, the HVPE technique still has the same inherent problems due to its growth on foreign substrates. Therefore, the growth of thick GaN using HVPE in general has to overcome two critical issues; firstly, to reduce the bending and cracking of initial GaN thick films (30-100 μm) on foreign substrates and secondly, to minimize the defect density of GaN.
The cracking of thick GaN film, due to the use of foreign substrates, depends on the growth and cooling conditions. The critical thickness for crack appearance in GaN can be improved from a typical value of 10-15 μm for GaN grown conventionally by the HVPE directly onto sapphire substrates, to 40-80 μm-thick crack-free layers by the use of reactively sputtered AlN buffer layers, or by employing ZnO buffer layers. However, even this thickness is not sufficient for safe handling during substrate separation. To further reduce the cracking in thicker GaN films in the initial growth, other growth techniques such as ELOG, growth on patterned substrates, re-growth with molten Ga interfacial layers, use of substrates better matched to GaN, and the use of thinned and mechanically weakened sapphire substrates have also been exploited.
To reduce defect density (mainly threading dislocations) and strain, and to improve the surface morphology of the thick GaN films grown by HVPE, various techniques have been employed, for example ELOG, growth under lower reactor pressure and growth with TiN intermediate layers, or deep inverse pyramid etch pits on weakened Si, GaAs and other III-V single crystal wafers. However, the growth processes using these techniques are tedious, time consuming and expensive. The GaN thus produced still has the major disadvantages of bending and undesired residual strain.
Various vapour deposition methods suitable for growing GaN materials are described in U.S. Pat. No. 6,413,627, U.S. Pat. No. 5,980,632, U.S. Pat. No. 6,673,149, U.S. Pat. No. 6,616,757, U.S. Pat. No. 4,574,093 and U.S. Pat. No. 6,657,232. Other publications relating to such methods include:    1. Handbook of Crystal Growth, Vol 3, edited by D. T. J. Hurle, Elsevier Science 1994.    2. R. F. Davis et al, ‘ Review of Pendeo-Epitaxial Growth and Characterization of Thin Films of GaN and AlGaN Alloys on 6H—SiC(0001) and Si(111) Substrates.’ MRS Internet J. Nitride Semicond. Res. 6, 14, 1 (2001).    3 M. Yoshiawa, A. Kikuchi, M. Mori, N. Fujita, and K. Kishino, ‘Growth of self-organised GaN nanostructures on Al2O3 (0001) by RF-radical source molecular beam epitaxy.’ Jpn. J. Appl. Phys., 36, L359 (1997).    4. K. Kusakabe, A. Kikuchi, and K. Kishino, ‘Overgrowth of GaN layer on GaN nano-columns by RF-molecular beam epitaxy.’ J. Crystl. Growth, 237-239, 988 (2002).    5. J. Su et al, ‘Catalytic growth of group III-nitride nanowires and nanostructures by metalorganic chemical vapor deposition.’ Appl. Phys. Lett., 86, 13105 (2005).    6. G. Kipshidze et al, ‘Controlled growth of GaN nanowires by pulsed metalorganic chemical vapor deposition.’ Appl. Phys. Lett., 86, 33104 (2005).    7. H. M. Kim et al, ‘Growth and characterization of single-crystal GaN nanorods by hydride vapor phase epitaxy.’ Appl. Phys. Lett., 81, 2193 (2002).    8. C. C. Mitchell et al., Mass transport in the epitaxial lateral overgrowth of gallium nitride.’ J. Cryst. Growth, 222, 144 (2001).    9. K. Hiramatsu, Epitaxial lateral overgrowth techniques used in group III nitride epitaxy.’ J. Phys: Condens, Matter, 13, 6961 (2001).    10. R. P. Strittmatter, ‘Development of micro-electromechnical systems in GaN’, PhD Thesis, California Institute of Technology, P. 92 (2003).